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Temperature Gauge Problem

F

Fordy20

Member
The truck I haul my tractors on the goose neck is a 1979 F-350 with the 351M engine. I've had it about six years. Had 15,000 miles when I bought it and 25,000 now. Always stored inside. The temperature gauge will sometimes show normal and sometimes way high. Might drive 30 miles on normal and then it will go up for no apparent reason. I replaced the sending unit and the thermostat (takes a special thermostat to close off the bypass to the water pump when the thermostat is closed so I have the right one). There's a transformer in the instrument panel and the gauge. Other gauges read normal and I did check when the gauge read high with a laser heat gun which showed normal. Would a faulty gauge act like that? Any ideas would be appreciated.
mvphoto69746.jpg
 
How is the wire connected to the sender? Is it a good tight, clean connection? Have you tried a new wire end/connecter?

This sounds like a poor/intermittent connection. I suppose that could be in the gauge, but I would think more likely out where the heat/vibration occurs.
 
"There's a transformer in the instrument panel"

Actually, it's an "instrument voltage regulator".

It slowly switches off and on and it's "duty cycle" "chops" the current to the oil, temp, and fuel gauges as a crude form of voltage regulation. The gauges are "thermal" and operate/change position slowly so you don't notice the gauge current switching on and off.

So if it gets "flaky" it should affect all the gauges, but I suppose it could affect the temperature gauge a little differently. ANYHOW, they DO fail from time to time, might not hurt to change it.

If it's not the IVR it IS possible that gauge is failing, also check that the IVR is well grounded, and there's no poor connection(s), intermittent shorts or damage in the gauge circuit board that could be affecting it.

<img src = "https://shop.broncograveyard.com/images/voltageregulator.JPG">

<img src = "https://i.imgur.com/HUQH7kZ.jpg">
 
(quoted from post at 18:28:06 02/09/21) How is the wire connected to the sender? Is it a good tight, clean connection? Have you tried a new wire end/connecter?

This sounds like a poor/intermittent connection. I suppose that could be in the gauge, but I would think more likely out where the heat/vibration occurs.
Click to expand...
he wiring on that truck is like new - couldn't make a better aftermarket connection. If that were the case I would think the gauge would fluctuate but it stays for some time in either higher or normal - not a quick change.
 
I'm sure you know this but I read somewhere that you are not supposed to use a sealant on the threads of the sending unit. It needs a good ground.
 
The voltage regulator on the back of the cluster powers 3 gauges (oil press, fuel level, eng temp) As you already know, the other gauges are fine but IVR's usually caused gauges to peg out-all 3. I have ran across several of these that had bad gauges do this same thing.

If you wanted to test the sender and wire to cluster and have a meter to measure Ohms. If the gauge is indicating hot, then the ohms should read 10 Ohms or somewhere close.

10 Ohms is hot
73 Ohms is cold
 
Could be the gauge, could be an intermittent short to ground on the sender wire.

Next time it happens, try disconnecting the wire from the sender. If the gauge stays high, the wire could be grounded somewhere.

Follow the wire, it probably goes through a harness plug at the firewall. Unplug the connector, see if there is corrosion, water contamination inside.
 
Thanks for the suggestions. Supposed to get a little cold snap so I’ll tackle it when it warms. Under cover now but not heated.
 
I have an 81 ford and the temp gauge never has worked right. It randomly reads high, in fact once I thought I was overheating and was pricing radiators until a little more research showed it was a bad gauge. I put in a mechanical temp gauge, now I have a second opinion. I have heard from other folks that them old ford gauges weren't so hot. Nice truck
 
Two thoughts;

1-You replaced the sensor, is it possible they gave you a sensor for an idiot light instead of one for a gauge?

2-Firewall to engine or battery to fender ground strap corroded or missing will result in all sorts of intermittent electrical gremlins.

Any flickering of headlights or dome lamp that you can recall?
 

My advice, do not go after the dash cluster its plastic if you do its a good bet its going to crumble up and no replacement can be found.

I have searched to world over I found one decent one there is a replacement offered aftermarket its never fit anything I have worked on... I will not take one out unless I have a good cluster in my hand just disconnecting the speed-o-meter cable will trash them... : ( About the only thing sorrier them them is a Dodge truck dash panel :lol:
 
I had a 68 gmc pickup in high school. Its gauges had no numbers, just low-med-high. After an engine rebuild I wanted to know more info so I put in a numbered oil press gauge using the 1/8 plastic oil line supplied with the gauge. Soon my gauges would all go funny now and then especially when I pulled on the lights. I mentioned it to a parts counter guy and he said reverse whatever you did. I thought "yah right". Well I was young and just starting my mechanic ventures and would find out that the original oil gauge had a steel line and was providing the ground for the dash. So, my long winded message is, check the ground.
 
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